The present invention relates to a gas turbine system and to a method for operating such a system.
In particular the present invention concerns a gas turbine system and related method of operating when using low Btu gaseous fuel, in that the system comprises a compressor unit for compressing air to compressed air, a combustion unit for combusting said fuel with the compressed air, both supplied to said combustion unit, thereby obtaining hot product gas, and an expansion turbine unit driven by the hot product gas supplied thereto and generating mechanical power.
Such a method and gas turbine system are well known in the art. For example in the article by J. Javetski, "A special report. The changing world of GAS TURBINES", Power, September 1978, Vol. 122, No. 9, discusses prior art and present developments in this field of application.
As to the case of combustion of low Btu gases and residual fuels additional modifications are discussed. Especially it is proposed to increase combustor diameters and lengths to accommodate with such fuels.
It will be clear that supplying low Btu gases to conventional gas turbine systems will require additional measures. Usually such systems are designed on being driven by combustion products originating from fuels having a high heating value per unit of mass, such as there are for natural gas or kerosine. In this context, fuels for such systems are referred to as high-, medium-, and low-Btu gaseous fuels as can be read in the article by J. G. Meier et al., "Development and Application of Industrial Gas Turbines for Medium-Btu Gaseous Fuels", Journal of Engineering for Gas Turbines and Power, January 1986, Vol. 108, pages 182-190. In particular low Btu-gases are defined as having a low heating value (LHV) when ranging from 3 to 8 MJ/Nm.sup.3 (equivalent with 75 to 200 Btu/scf).
Instabilities have to be expected when operating such systems with low-Btu gases as fuel supplied to the combustion unit, for example, because of varying heating values and/or varying mass flow values. Blast furnace gas is used as a low-BTU gas.
In order to remedy the shortcomings of the system addressed above, the Meier article discloses some measures as can be seen in table 7 thereof. Moreover, from this article it can be read that mixing such fuels with air prior to injection is discouraged because of danger of auto-ignition.
From U.S. Pat. No. 4,677,829 it is also known to feed a gas turbine with fuel only consisting of low-Btu gases. In this patent inlet air to the compression unit is heated to increase the temperature of the inlet air, thereby decreasing its density and consequently its mass flow. Thus, although low-Btu gas is employed, a constant power output is obtained.
From the above a plurality of measures in order to operate conventional gas turbine systems, more or less modified, with only low-Btu gaseous fuel are known. However, substantial modifications as to the respective units or substantially increased complexity of the system have appeared to be necessary. Especially as to countries where low-Btu gaseous originating from gasifying biomass is present the above measures will be problematic.